preventing aluminum galvinazation

J

JackB

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I plan to mount some old bronze ship lights on top of pressure treated wood. To do so I had 1/4" aluminum sheets fabricated to first mount on the top of the pressure treated posts. The marine lights will be mounted [bolted] to the aluminum plates.
My first inclination was to be sure to use aluminum screws when mounting the aluminum plate to avoid the galvanic action but then I got thinking that the problem may be with the aluminum screw threads and the chemical treated wood. Someone else suggested that I could use regular coated decking screws because the coating would isolate the two metals..
Lastly since the flat surface of the bronze lights will sit on top of the aluminum plate which will be painted does that require a gasket or will the paint be good enough to isolate the metals?
 
I'd mount the bronze ship lights directly to the wood with bronze or stainless steel screws/bolts; and forget the aluminum.

Bronze and aluminum don't like each other much and mounting bronze lights on aluminum plates is IMO asking for trouble. I wouldn't trust any kind of "gasket" to separate them.
 
a good suggestion to not use aluminum. Formerly treated wood contained copper which would be a good with bronze. I dont know whats in it today.
 
First, are these being mounted on a boat? Trailer What? In or near salt water?

Or are they decorative lighting for your home or dock?

Forget the aluminum. You can buy bronze screws and you can have bronze plates made if you need mounting plates.

Another choice might be plastic (Lexan, etc.) for the mounting plates and coated deck screws to mount the plates to the wood.

A little more information and you'll get better suggestions.
 
Attach the lights to Starboard bases, attach the Starboard bases to the posts.
 
OK here is some more information and a revised plan thanks to the concerns about the aluminum. I do need to use the aluminum because I had them fabricated inexpensively and bronze would have been difficult to obtain. These are being mounted on my deck posts near the Ocean. Mounted on the top of 6x6" treated deck posts. In this order. First on top is treated pre fab decorative wooden post cap. I now plan to use a 1/2" thick by 3" diameter plastic sewer pipe section as a spacer on top of the decorative cap. The aluminum plate is on top of the spacer and 4 3" screws will go thru the plate and the decorative cap into the top of the post. Those screws will be inside the plastic spacer, out of the weather. I would like to use 3" coated deck screws as they are readily available. They are the screws that had me concerned as they will be contacting the aluminum. NEXT on top of the aluminum plate, thanks to you guys and the galvanization concerns, I will place a second 1/2" plastic section [ my wife approved the design] and finally the bronze light base will sit on that plastic section with stainless bolts going down through holes in the aluminum plate. The plate, by the way, was needed initially so I had enough room to bolt the lights down otherwise the nuts hit the wood post cap. Each light has 4 mounting holes. How does this all sound now?
Just an aside, these lights, red and clear lense were picked up at a marine salvage place. The lenses are 360 degrees red or clear as mentioned and are massively built. The lenses are held on with 4 swing bolts so bulbs can be changed and are about 5 inches in diameter and the lights are around 8 inches tall. Each has a special waterproof plastic screw fitting for the electric line. Some have only one such fitting and a few others have as many as 3. One of the lights has 6 bulbs in it of 2 differnt sizes. Does anyone know why and how that worked on a ship?
One last question about those spacers. I can either put caulk on the edges of those plastic spacers to seal the insides up or file round notches in the bottom of each one to allow any condensate to drain and air to get in. Which method is preferred? Thanks again for the assistance. Amazing how what I thought would be a simple project got a little complicated.
 
Use plastic washers to insulate the screws from the metal. I would fully seal the pieces instead of adding drain holes unless water can get in from the top. A small air hole way underneath might help eliminate air pressire changes affecting the seal.
 
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